This invention pertains to a method and an apparatus for the elimination of color from multi-color image documents.
For character recognition purposes it is very advisable if not vital, to eliminate or to drop-out all image information which is redundant, such as for example the pre-printed form. Only the filled-in information is to be used for the optical recognition process. To facilitate the forms drop-out, the banking industry uses colored forms printed in colors such as red, green, yellow, and blue.
Usually optical readers of multi-font recognition capability in the banking and retail industries are equipped with optical drop-out color filters. These filters are placed in the optical path on a mechanical filter bank. It is known in the art to shift this filter bank by mechanical means such that the right filter is placed in the optical path, i.e. red, green, or blue color filter, for example, is placed in the optical path. The operator of such optical reading equipment has first to analyze the actual document in order to assess which drop-out color is to be used. Then the applicable color filter has to be manually adjusted using a slider in the optical path. So, for a red pre-printed form, a red color filter will be used. Due to the rules of physics the contrast of the pre-printed areas will strongly be reduced, or will be dropped-out but the filled-in information which is not printed and written in the same color, in this example red, remains untouched on the document. In most cases the contrast of the filled-in information is increased related to the background color. This afore-mentioned described procedure has to be performed for each color depending on the color of the pre-printed form.
It is known that the same effect is achieved with color lamps and color cameras if only one color is used at a time.
The above described simple methods fail totally if multi-color background is used, as for example especially in euro-cheques. If there for example a red drop-out color is used, then the red lines are dropped-out but not the green and the blue lines. In case of a blue filter, the blue lines are dropped-out but not the red and the green ones. In case of a green filter, the green lines are dropped-out but not the red and the blue ones. Thus, the background is not totally dropped-out.
There are other types of forms drop-out methods such as forms subtraction and spatial filtering methods. In forms subtraction the image of the empty pre-printed form is in a way subtracted from the filled-in form of the same type. In this method a mask matching method is used. This method has certain sensitivity regarding form shrinkage or expansion, form rotation, tolerances in the printing, and it shows a strong dependency on resolution. As a result usually there are still some parts of the background remaining on the document.
In forms drop-outs by spatial filtering methods there are used spatial frequency filters, logical filters and density filters, as described in many books and articles about digital image processing. These filters can reduce but not really drop-out the background without destroying the filled-in information to a certain extent, so-called erosion of the information, or with parts of the background left, so-called artifacts.
As a conclusion it can be said that traditional color drop-out methods as described before for mono color pre-printed forms are most efficient but fail for multi-color documents.
Therefore the objects of the present invention are to provide a method and an apparatus which efficiently remove or eliminate multi-color images contained on documents without using the handling of mechanical filter adjustment or the like.
This object as well as other objects are solved advantageously basically by applying the features described herein. Further advantageous embodiments are also described.
In accordance with the present invention color picture information is used, then for every single picture element all the image signals of all colors, normally the three colors red, green, and blue are analyzed, and finally by a special electronic or logical set-up automatically only the image signal of that color which has the minimum contrast relative to the document background are selected. For the analyzing purpose the color information is concurrently provided for every picture element of the image.
By this inventive basic solution an automatic picture element related color drop-out or elimination from the background is provided. This is performed without using any mechanical adjustments of filters as in the prior art.
In accordance with an advantageous further development of the present invention the color image signals are weighted and combined to a secondary output to provide a grey scale signal for archiving purposes of the image document.